A mass grave with over 100 bodies found in the town of Hammam al-Alil was one of several ISIS killing grounds, Shamdasani said, citing information gleaned from sources on the ground including a man who played dead during a mass execution.

The public executions were being carried out for “treason and collaboration” with Iraqi forces trying to recapture the city, or for the use of banned mobile phones or desertion.

People with explosive belts, possibly teenagers or young boys, were being deployed in the alleys of Old Mosul, while abducted women were being "distributed" to fighters or told they would be used to accompany Islamic State convoys, she said.